Composite color-plus-clear coating utilizing carbamate-functional polymer composition in the clearcoat

ABSTRACT

A method of producing an article with a color-plus-clear composite coating is described. The method comprises the steps of applying a colored coating composition to a substrate, and applying a clear coating composition over the colored coating composition, wherein the clear coating composition is a curable coating composition comprising: 
     (a) a first component comprising a polymer backbone having appended thereto at least one carbamate functional group, and 
     (b) a second component comprising a compound having a plurality of functional groups that are reactive with said carbamate group.

This is a continuation of application Ser. No. 07/965,577 filed on Oct.23, 1992, now U.S. Pat. No. 5,356,669.

FIELD OF THE INVENTION

This invention relates to composite color-plus-clear coatings andmethods, especially compositions for the clearcoat of such coatings.

BACKGROUND OF THE INVENTION

Color-plus-clear composite coatings are widely utilized in the coatingsart. They are particularly desirable where exceptional gloss, depth ofcolor, distinctness of image, or special metallic effects are desired.The automotive industry has made extensive use of color-plus-clearcomposite coatings for automotive body panels. Such coatings, however,require an extremely high degree of clarity in the clearcoat to achievethe desired visual effect. As such, the clearcoat of a color-plus-clearcomposite coating is especially susceptible to a phenomenon known asenvironmental etch. Environmental etch manifests itself as spots ormarks on or in the clear finish of the coating that often cannot berubbed out.

It is often difficult to predict the degree of resistance toenvironmental etch that a clearcoat will exhibit. Many coatingcompositions known for their durability and/or weatherability when usedin exterior paints, such as high-solids enamels, do not provide thedesired level of resistance to environmental etch when used as theclearcoat of a color-plus-clear composite coating.

Many compositions have been proposed for use as the clearcoat of acolor-plus-clear composite coating, such as polyurethanes, acid-epoxysystems and the like. However, many prior art systems suffer fromdisadvantages such as coatability problems, compatibility problems withthe pigmented basecoat, solubility problems. Moreover, very few one-packcoating compositions have been found that provide satisfactoryresistance to environmental etch, especially in the demandingenvironment of automotive coatings. Thus, there exists a continuing needfor curable coating compositions that provide satisfactory resistance toenvironmental etch when used as the clearcoat of a color-plus-clearcomposite coating.

SUMMARY OF THE INVENTION

It has now been discovered that carbamate-functional acrylic polymerscan be used in the clearcoat composition of a color-plus-clear compositecoating. Thus, according to the present invention, there is provided amethod of applying a color-plus-clear composite coating comprising thesteps of applying a colored coating composition to a substrate, andapplying a clear coating composition over the colored coatingcomposition, wherein the clear coating composition is a curable coatingcomposition comprising:

(a) a first component comprising a polymer backbone having appendedthereto at least one carbamate functional group, and

(b) a second component comprising a compound having a plurality offunctional groups that are reactive with said carbamate group.

The composite coating, when cured, provides a hard but flexible,durable, attractive clearcoat finish that is highly resistant toenvironmental etch. The clearcoat composition can be effectively appliedas a one-pack system without the necessity of mixing reactive materialsjust prior to application as in a two-pack system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polymer component (a) used in the composition of the invention canbe prepared in a variety of ways. One way to prepare such polymers is toprepare an acrylic monomer having a carbamate functionality in the esterportion of the monomer. Such monomers are well-known in the art and aredescribed, for example in U.S. Pat. Nos. 3,479,328, 3,674,838,4,126,747, 4,279,833, and 4,340,497, the disclosures of which areincorporated herein by reference. One method of synthesis involvesreaction of a hydroxy ester with urea to form the carbamyloxycarboxylate (i.e., carbamate-modified acrylic). Another method ofsynthesis reacts an α,β-unsaturated acid ester with a hydroxy carbamateester to form the carbamyloxy carboxylate. Yet another techniqueinvolves formation of a hydroxyalkyl carbamate by reacting a primary orsecondary amine or diamine with a cyclic carbonate such as ethylenecarbonate. The hydroxyl group on the hydroxyalkyl carbamate is thenesterified by reaction with acrylic or methacrylic acid to form themonomer. Other methods of preparing carbamate-modified acrylic monomersare described in the art, and can be utilized as well. The acrylicmonomer can then be polymerized along with otherethylenically-unsaturated monomers, if desired, by techniques well-knownin the art.

An alternative route for preparing the polymer (a) used in thecomposition of the invention is to react an already-formed polymer suchas an acrylic polymer with another component to form acarbamate-functional group appended to the polymer backbone, asdescribed in U.S. Pat. No. 4,758,632, the disclosure of which isincorporated herein by reference. One technique for preparing polymersuseful as component (a) involves thermally decomposing urea (to give offammonio and HNCO) in the presence of a hydroxy-functional acrylicpolymer to form a carbamate-functional acrylic polymer. Anothertechnique involves reacting the hydroxyl group of a hydroxyalkylcarbamate with the isocyanate group of an isocyanate-functional acrylicor vinyl monomer to form the carbamate-functional acrylic.Isocyanate-functional acrylics are known in the art and are described,for example in U.S. Pat. No. 4,301,257, the disclosure of which isincorporated herein by reference. Isocyanate vinyl monomers arewell-known in the art and include unsaturated m-tetramethyl xyleneisocyanate (sold by American Cyanamid as TMI®). Yet another technique isto react the cyclic carbonate group on a cyclic carbonate-functionalacrylic with ammonia in order to form the carbamate-functional acrylic.Cyclic carbonate-functional acrylic polymers are known in the art andare described, for example, in U.S. Pat. No. 2,979,514, the disclosureof which is incorporated herein by reference. A more difficult, butfeasible way of preparing the polymer would be to trans-esterify anacrylate polymer with a hydroxyalkyl carbamate.

The polymer (a) will generally have a molecular weight of 2000-20,000,and preferably from 4000-6000. Molecular weight can be determined by theGPC method using a polystyrene standard. The carbamate content of thepolymer, on a molecular weight per equivalent of carbamatefunctionality, will generally be between 200 and 1500, and preferablybetween 300 and 350. The glass transition temperature, T_(g), ofcomponents (a) and (b) can be adjusted to achieve a cured coating havingthe T_(g) for the particular application involved. The average T_(g) ofunreacted components (a) and (b) should be between 10° C. and 80° C.,with the individual T_(g) 's being adjusted to achieve optimumperformance.

The polymer component (a) can be represented by the randomly repeatingunits according to the following formula: ##STR1##

In the above formula, R₁ represents H or CH₃. R2 represents H, alkyl,preferably of 1 to 6 carbon atoms, or cycloalkyl, preferably up to 6ring carbon atoms. It is to be understood that the terms alkyl andcycloalkyl are to include substituted alkyl and cycloalkyl, such ashalogen-substituted alkyl or cycloalkyl. Substituents that will have anadverse impact on the properties of the cured material, however, are tobe avoided. For example, ether linkages are thought to be susceptible tohydrolysis, and should be avoided in locations that would place theether linkage in the crosslink matrix. The values x and y representweight percentages, with x being 10 to 90% and preferably 40 to 60%, andy being 90 to 10% and preferably 60 to 40%.

In the formula, A represents repeat units derived from one or moreethylenically unsaturated monomers. Such monomers for copolymerizationwith acrylic monomers are known in the art. They include alkyl esters ofacrylic or methacrylic acid, e.g., ethyl acrylate, butyl acrylate,2-ethylhexyl acrylate, butyl methacrylate, isodecyl methacrylate,hydroxyethyl methacrylate, hydroxypropyl acrylate, and the like; andvinyl monomers such as unsaturated m-tetramethyl xylene isocyanate (soldby American Cyanamid as TMI®), styrene, vinyl toluene and the like.

L represents a divalent linking group, preferably an aliphatic of 1 to 8carbon atoms, cycloaliphatic, or aromatic linking group of 6 to 10carbon atoms. Examples of L include ##STR2## --(CH₂)--, --(CH₂)₂ --,--(CH₂)₄ --, and the like. In one preferred embodiment, --L-- isrepresented by --COO--L'-- where L' is a divalent linking group. Thus,in a preferred embodiment of the invention, the polymer component (a) isrepresented by randomly repeating units according to the followingformula: ##STR3##

In this formula, R₁, R₂, A, x, and y are as defined above. L' may be adivalent aliphatic linking group, preferably of 1 to 8 carbon atoms,e.g., --(CH₂)--, --(CH₂)₂ --, --(CH₂)₄ --, and the like, or a divalentcycloaliphatic linking group, preferably up to 8 carbon atoms, e.g.,cyclohexyl, and the like. However, other divalent linking groups can beused, depending on the technique used to prepare the polymer. Forexample, if a hydroxyalkyl carbamate is adducted onto anisocyanate-functional acrylic polymer, the linking group L' wouldinclude an --NHCOO-- urethane linkage as a residue of the isocyanategroup.

The composition of the invention is cured by a reaction of thecarbamate-functional polymer component (a) with a component (b) that isa compound having a plurality of functional groups that are reactivewith the carbamate groups on component (a). Such reactive groups includeactive methylol or methylalkoxy groups on aminoplast crosslinking agentsor on other compounds such as phenol/formaldehyde adducts, isocyanategroups, siloxane groups, cyclic carbonate groups, and anhydride groups.Examples of (b) compounds include melamine formaldehyde resin (includingmonomeric or polymeric melamine resin and partially or fully alkylatedmelamine resin), urea resins (e.g., methylol ureas such as ureaformaldehyde resin, alkoxy ureas such as butylated urea formaldehyderesin), polyanhydrides (e.g., polysuccinic anhydride), and polysiloxanes(e.g., trimethoxy siloxane). Aminoplast resin such as melamineformaldehyde resin or urea formaldehyde resin are especially preferred.Even more preferred are aminoplast resins where one or more of the aminonitrogens is substituted with a carbamate group for use in a processwith a curing temperature below 150° C., as described in theconcurrently-filed U.S. patent application entitled"Carbamate-Defunctionalized Aminoplast Curing for Polymer Compositions"in the names of John W. Rehfuss and Donald L. St. Aubin.

A solvent may optionally be utilized in the clearcoat composition usedin the practice of the present invention. Although the composition usedaccording to the present invention may be utilized, for example, in theform of substantially solid powder, or a dispersion, it is oftendesirable that the composition is in a substantially liquid state, whichcan be accomplished with the use of a solvent. This solvent should actas a solvent with respect to both the carbamate-functional polymer (a)as well as the component (b). In general, depending on the solubilitycharacteristics of components (a) and (b), the solvent can be anyorganic solvent and/or water. In one preferred embodiment, the solventis a polar organic solvent. More preferably, the solvent is a polaraliphatic solvents or polar aromatic solvents. Still more preferably,the solvent is a ketone, ester, acetate, aprotic amide, aproticsulfoxide, or aprotic amine. Examples of useful solvents include methylethyl ketone, methyl isobutyl ketone, m-amyl acetate, ethylene glycolbutyl ether-acetate, propylene glycol monomethyl ether acetate, xylene,N-methylpyrrolidone, or blends of aromatic hydrocarbons. In anotherpreferred embodiment, the solvent is water or a mixture of water withsmall amounts of aqueous co-solvents.

The clearcoat composition used in the practice of the invention mayinclude a catalyst to enhance the cure reaction. For example, whenaminoplast compounds, especially monomeric melamines, are used ascomponent (b), a strong acid catalyst may be utilized to enhance thecure reaction. Such catalysts are well-known in the art and include, forexample, p-toluenesulfonic acid, dinonylnaphthalene disulfonic acid,dodecylbenzenesulfonic acid, phenyl acid phosphate, monobutyl maleate,butyl phosphate, and hydroxy phosphate ester. Other catalysts that maybe useful in the composition of the invention include Lewis acids, zincsalts, and tin salts.

In a preferred embodiment of the invention, the solvent is present inthe clearcoat composition in an amount of from about 0.01 weight percentto about 99 weight percent, preferably from about 10 weight percent toabout 60 weight percent, and more preferably from about 30 weightpercent to about 50 weight percent.

Coating compositions can be coated on the article by any of a number oftechniques well-known in the art. These include, for example, spraycoating, dip coating, roll coating, curtain coating, and the like. Forautomotive body panels, spray coating is preferred.

Pigmented basecoat compositions for such composite coatings arewell-known in the art, and do not require explanation in detail herein.Polymers known in the art to be useful in basecoat compositions includeacrylics, vinyls, polyurethanes, polycarbonates, polyesters, alkyds, andpolysiloxanes. Preferred polymers include acrylics and polyurethanes. Inone preferred embodiment of the invention, the basecoat composition alsoutilizes a carbamate-functional acrylic polymer. Basecoat polymers arepreferably crosslinkable, and thus comprise one or more type ofcross-linkable functional groups. Such groups include, for example,hydroxy, isocyanate, amine, epoxy, acrylate, vinyl, silane, andacetoacetate groups. These groups may be masked or blocked in such a wayso that they are unblocked and available for the cross-linking reactionunder the desired curing conditions, generally elevated temperatures.Useful cross-linkable functional groups include hydroxy, epoxy, acid,anhydride, silane, and acetoacetate groups. Preferred cross-linkablefunctional groups include hydroxy functional groups and amino functionalgroups.

Basecoat polymers may be self-cross-linkable, or may require a separatecross-linking agent that is reactive with the functional groups of thepolymer. When the polymer comprises hydroxy functional groups, forexample, the cross-linking agent may be an aminoplast resin, isocyanateand blocked isocyanates (including isocyanurates), and acid or anhydridefunctional cross-linking agents.

After an article is coated with the above-described layers, thecomposition is subjected to conditions so as to cure the coating layers.Although various methods of curing may be used, heat-curing ispreferred. Generally, heat curing is effected by exposing the coatedarticle to elevated temperatures provided primarily by radiative heatsources. Curing temperatures will vary depending on the particularblocking groups used in the cross-linking agents, however they generallyrange between 93° C. and 177° C., and are preferably between 121° C. and141° C. The curing time will vary depending on the particular componentsused, and physical parameters such as the thickness of the layers,however, typical curing times range from 15 to 60 minutes.

The invention is further described in the following examples.

PREPARATION 1--CARBAMATE-FUNCTIONAL ACRYLIC

A three-necked 5-1 round bottom flask was fitted with an agitator at thecenter neck and a thermal couple at one of the side necks to monitor thereaction temperature. A nitrogen purge line was also fed through thisneck. The second side neck was fitted with a Claissen adaptor and watercooled condenser.

198 g Urethane-grade mixed aromatics solvent (Solvesso® 100) and 225 gurethane-grade toluene were charged to the flask. The mixture wasagitated and heated to reflux with a nitrogen purge. As the mixturereached reflux temperature, 127° C., the nitrogen purge wasdiscontinued.

923 g TMI® (unsaturated m-tetramethyl xylene isocyanate, AmericanCyanamid), 692 g ethyl hexyl acrylate and 269 g of a 50% solution oft-butyl peracetate in odorless mineral spirits were charged to aseparate container. This mixture was pumped to the refluxing solventsover a period of 3.5 hour. At the conclusion of this first feed, asecond addition of 27 g of the t-butyl peracetate solution and 27 gurethane grade mixed aromatics were charged over 30 minutes. 8.2 gUrethane-grade mixed aromatics was flushed through the pump and into thereaction mixture after the second initiator feed. The reaction mixturewas then held at reflux, 135° C. for one hour.

After this hold period, the batch was cooled to 70° C. 1.1 g Dibutyltindilaurate was charged and mixed into the batch for five minutes. At thispoint, 565 g hydroxypropyl carbamate was charged to the reaction mixtureover 30 minutes. The batch was then slowly heated to 100° C. and held atthis temperature until isocyanate functionality had disappeared asdetermined by infrared spectroscopy or titration. Upon the disappearanceof the isocyanate, 852 g monobutyl ether of ethylene glycol was chargedto the vessel and allowed to homogenize. The heat to the reaction wasturned off and the carbamate functional acrylic was removed from thevessel.

PREPARATION 2--CARBAMATE-MODIFIED MELAMINE

A three-necked 5-1 round-bottomed flask was fitted with a vacuum sealedagitator at the center neck and a thermocouple at a side neck to monitorthe reaction temperature. The second side neck as temporarily fittedwith a water cooled condensor. Vacuum was applied through a collectingvessel and supercooled condensor via this side neck of the reactionflask.

1708 g Hexamethoxylated monomeric melamine and 1044 g butyl carbamatewere charged to the flask. The mixture was homogenized with agitationwhile heating slowly to 60° C. As the mixture reached 60° C., 1.2 gdodecylbenzyl sulfonic acid was charged to the vessel. The condensor wasremoved and the flask fitted to the vacuum set-up. The mixture washeated to 100° C. at a rate of 1° C./min. When the mixture reached 70°C., 15-20" vacuum was applied. The methanol was collected as itcondensed in the supercooled condensor. A stoichiometric amount ofmethanol, 279 g, was removed in 2.5 hours at 25" vacuum and 100° C.After this amount was removed, the heat and vacuum were discontinued.The vessel was charge with 433 g xylene, homogenized, andcarbamate-modified melamine separated from the mixture.

EXAMPLE 1

A clear coating composition was prepared by combining the followingmaterials:

665 g carbamated acrylic (Preparation 1)

167 g carbamated melamine (Preparation 2)

345 g butyl acetate

44 g Exxate® 800 (methyl octoate isomers)

19 g Tinuvin® 384B

6 g Tinuvin® 123

12 g 25% active oxizolidine blocked dodecylbenzyl sulfonic acid

The coating composition was sprayed over steel panels that had beenpreviously sprayed with an acrylic pigmented base coat and flashed.Viscosity was adjusted to seconds with butyl acetate. The panels werebaked 10 minutes at 82° C. and 20 minutes at 132° C.

Film builds:

basecoat 15 μm

clearcoat 51 μm

Tukon hardness 13.5

MEK rubs 200, slight scoring

The panel of Example i was subjected to 16 weeks of severe weatheringconditions in Jacksonville, Fla., and exhibited significantly reducedenvironmental etch versus comparison panels coated having clearcoats ofhydroxyl-functional acrylic polymer cross-linked with melamine.

EXAMPLE 2

A clear coating composition was prepared by combining the followingmaterials:

184 g carbamated acrylic (Preparation 1)

60 g hexamethoxylated monomeric melamine

130 g butyl acetate

14 g butyl cellosolve acetate

6 g Tinuvin® 384B

1.9 g Tinuvin® 123

3.8 g 25% active oxizolidine blocked dodecylbenzyl sulfonic acid

The coating composition was sprayed over steel panels that had beenpreviously sprayed with an acrylic pigmented basecoat and flashed.Viscosity was adjusted to 20 seconds with butyl acetate. The panels werebaked 10 minutes at 82° C. and 20 minutes at 132° C.

Film builds:

basecoat 15 μm

clearcoat 58 μm

The panel of Example 2 was subjected to 16 weeks of severe weatheringconditions in Jacksonville, Fla., and exhibited significantly reducedenvironmental etch versus comparison panels coated having clearcoats ofhydroxyl-functional acrylic polymer cross-linked with melamine.

PREPARATION 3--CARBAMATE-FUNCTIONAL ACRYLIC

A three-necked 5-1 round bottom flask was fitted with an agitator at thecenter neck and a thermal couple to monitor the reaction temperature atone of the side necks. A nitrogen purge/sparge line was also fed throughthis neck. The second side neck was fitted with a Claissen adaptor andwater-cooled condenser.

235 g Xylene and 356 g amyl acetate were charged to the flask. Themixture was agitated and heated to reflux with a nitrogen purge. As themixture reached reflux, 143° C., the nitrogen purge was discontinued.301 g Styrene, 196 g ethylhexyl acrylate, 337 g ethylhexyl methacrylate445 g hydroxyethyl methacrylate, 226 g cyclohexyl methacrylate, 123 g ofa 50% solution of t-butyl peracetate in odorless mineral spirits, and116 g xylene were charged to a separate container. This mixture waspumped to the refluxing solvent over a period of four hours. At theconclusion of this feed, 35 g xylene was added through the pump and intothe reaction mixture. The reaction mixture was held at reflux, 140° C.,for one hour.

The mixture was cooled to 120° C. and charged with 205 g urea. Thetemperature dropped as the urea dissolved. The reaction mixture wasslowly heated to 150° C. and held for the remainder of the synthesis.

The vessel was then charged with 2 g of King Industry catalyst Nacure®XP-348 (metal carbalate). At this point, the reaction was sparged withnitrogen to facilitate the evacuation of ammonia formed from the thermaldecomposition of the urea.

Incremental additions of the catalyst (0.5 g) were added once an hour.The reaction was monitored for the disappearance of hydroxyl bytitration. When no hydroxyl was detected by titration, the nitrogensparge and heat were cut, and 560 g methyl isobutyl ketone was added tothe mixture. The mixture was homogenized, followed by separation of thepolymer.

EXAMPLE 3

A coating composition was formed by blending 50 g of thecarbamate-functional acrylic from Preparation 3, 7.7 g hexamethoxylatedmonomeric melamine, and 0.6 g oxizolidine-blocked dodecylbenzyl sulfonicacid. The composition was coated onto a glass plate, followed by vacuumdrawdown to form an 200 μm-thick layer. The cured coating was baked at132° C. for 30 minutes. The coating passed a test of 200 MEK rubs.

The invention has been described in detail with reference to preferredembodiments thereof. It should be understood, however, that variationsand modifications can be made within the spirit and scope of theinvention.

What is claimed is:
 1. A method of producing a color-plus-clearcomposite coating on a substrate comprising the steps of applying acolored coating composition to said substrate, and applying a clearcoating composition over the colored coating composition, wherein theclear coating composition is a curable coating composition comprising,in an organic solvent medium:(a) a first component comprising a polymerbackbone having appended thereto at least one carbamate functionalgroup, said first component represented by randomly repeating unitsaccording to the formula: ##STR4## R₁ represents H or CH₃, R₂ representsH, alkyl, or cycloalkyl,L represents a divalent linking group, Arepresents repeat units derived from one or more ethylenicallyunsaturated monomers, x represents 10 to 90 weight %, and y represents90 to 10 weight %, and (b) a second component comprising a compoundhaving a plurality of functional groups that are reactive with saidcarbamate group.
 2. A method according to claim 1 wherein saidethylenically unsaturated monomers comprise a carbamate group.
 3. Amethod according to claim 1 wherein said ethylenically unsaturatedmonomers comprise one or more acrylic monomers.
 4. A method according toclaim 3 wherein said acrylic monomers comprise a carbamate group.
 5. Amethod according to claim 1 wherein 10-90% of said ethylenicallyunsaturated monomers are acrylic monomers.
 6. A method according toclaim 1 wherein component (b) is selected from the group consisting ofaminoplast resin, polysiloxanes, polyanhydrides, and compounds having aplurality of active methylol functional groups.
 7. A method according toclaim 1 wherein component (b) is an aminoplast resin.
 8. A methodaccording to claim 7 wherein said aminoplast resin is melamineformaldehyde resin.
 9. A method according to claim 8 wherein saidmelamine formaldehyde resin is fully or partially alkylated.
 10. Amethod according to claim 1 wherein R₁ represents CH₃.
 11. A methodaccording to claim 1 wherein x represents 40 to 60 weight % and yrepresents 60 to 40 weight %.
 12. A method according to claim 1 wherein--L-- is represented by the formula --COO--L'-- where L' is a divalentlinking group.
 13. A method according to claim 1 wherein R₂ representsH.
 14. A method according to claim 1 wherein R₂ represents alkyl orcycloalkyl.
 15. A method according to claim 1 wherein said substrate isan automobile body panel.